1. Field of the Invention
The present invention relates to a diversity reception method and diversity receiver used to receive a packet made up of a preamble field and data field.
2. Description of the Prior Art
According to a conventional method as disclosed in Japanese Unexamined Patent Publication No. 63-158922, an antenna branch having a maximum reception signal power upon reception of a preamble field is selected, and a data field is demodulated based on a reception signal from the selected antenna branch.
Japanese Unexamined Patent Publication No. 10-28107 discloses a diversity receiver shown in the block diagram of FIG. 1 as a diversity receiver for receiving a packet made up of a preamble field and data field having undergone direct spread modulation. FIG. 1 shows a receiver using two antenna branches.
In FIG. 1, reference numerals 1000-1 and 1000-2 denote antenna branches; 1001, a switch (SW); 1002, a gate; 1003, a demodulation circuit; 1004, an antenna switching control circuit; and 1005, an output terminal.
This diversity receiver cannot predict packet arrival. For this reason, the antenna branches 1000-1 and 1000-2 are selected by the switch 1001 at a predetermined period, and a reception signal from the selected antenna branch is supplied to the antenna switching control circuit 1004 and gate 1002.
The antenna switching control circuit 1004 calculates the correlation between an input signal and spreading code. When the peak value of the correlation is equal to or higher than a predetermined threshold, a packet arrival detection pulse is output to the switch 1001 and gate 1002. Upon reception of this pulse, the switch 1001 stops periodic switching operation and holds the current state.
Upon reception of the pulse reception detection pulse, the gate 1002 opens its gate to supply an input signal to the demodulation circuit 1003. The demodulation circuit 1003 demodulates the input signal and outputs the demodulation result to the output terminal 1005 on the basis of the signal via the gate 1002. These methods can be classified into a pre-detection antenna switching diversity method, which can be implemented by a simple receiver.
According to another method, a diversity receiver which provides more excellent characteristics uses a signal obtained after detecting a reception signal from each antenna branch, though the structure of the receiver is complicated.
For example, Japanese Unexamined Patent Publication No. 03-214819 discloses a post-detection selection diversity method of selecting an antenna branch using a signal after detection.
This method is shown in FIG. 2. FIG. 2 shows a receiver using two antenna branches 1000-1 and 1000-2. Reception signals from the antenna branches 1000-1 and 1000-2 are detected by detection circuits 1100-1 and 1100-2, and channel impulse responses of reception signals are estimated by channel impulse response estimation circuits 1101-1 and 1101-2.
Based on the channel impulse responses estimated by the channel impulse response estimation circuits, an antenna branch receiving a reception signal, which has the smallest distortion, and supplying the channel impulse response estimated from the reception signal is selected by a selection control circuit 1102 and switches 1103 and 1104. The data field is equalized and demodulated by an equalizer 1105.
In FIG. 2, the receiver comprises the antenna branches 1000-1 and 1000-2, detection circuits 1100-1 and 1100-2, channel impulse response estimation circuits 1101-1 and 1101-2, selection control circuit 1102, switches 1103 and 1104, and equalizer 1105. Reference numeral 1106 denotes an output terminal.
Further, Japanese Unexamined Patent Publication No. 08-163103 discloses a post-detection combining diversity method of detecting a reception signal from each antenna branch and combining the post-detection signals.
This method is shown in FIG. 3. FIG. 3 shows a receiver using two antenna branches 1000-1 and 1000-2. Reception signals from the antenna branches 1000-1 and 1000-2 are respectively detected by detection circuits 1100-1 and 1100-2, and the channel impulse responses of the reception signals are estimated by channel impulse response estimation circuits 1101-1 and 1101-2.
Branch metrics for the reception signals from the respective antenna branches are calculated by branch metric calculation circuits 1201-1 and 1201-2 on the basis of the reception signals from the respective antenna branches and their estimated channel impulse responses. The calculated branch metrics are combined by a combining circuit 1202, and the data field is demodulated by a Viterbi equalizer 1203 based on the synthesized value.
In FIG. 3, the receiver comprises the antenna branches 1000-1 and 1000-2, detection circuits 1100-1 and 1100-2, channel impulse response estimation circuits 1101-1 and 1101-2, branch metric calculation circuits 1201-1 and 1201-2, synthesis circuit 1202, and Viterbi equalizer 1203. Reference numeral 1204 denotes an output terminal.
In the conventional pre-detection antenna switching diversity method shown in FIG. 1, even if packet arrival is detected based on a signal from a given antenna branch, a signal received by another antenna branch may have a larger reception power.
However, shortening the preamble field to increase the transmission efficiency limits the time required to confirm the reception state of other antenna branches. Thus, if the number of antenna branches is increased to improve reception characteristics, the antenna branch is not always switched to an optimum one.
By applying to packet transfer the post-detection selection diversity method and post-detection combining diversity method shown in FIGS. 2 and 3, the number of antenna branches can be increased while the length of the preamble field remains the same. However, these methods cannot predict packet arrival, so demodulators equal in number to antenna branches must always operate. This leads to large power consumption of the receiver.
The present invention has been made in consideration of the above situation, and has as its object to provide a diversity reception method and diversity receiver capable of suppressing the power consumption of the receiver and increasing the number of antenna branches to improve reception characteristics even in communicating a packet having a short preamble field.
To achieve the above object, according to the first aspect of the present invention, there is provided a diversity reception method for a diversity receiver having n (integer not smaller than 2) antenna branches for receiving a packet made up of a preamble field and data field, comprising the steps of switching the n antenna branches to detect packet arrival when no packet arrival is detected, stopping switching the n antenna branches when packet arrival is detected, and performing reception operation by demodulating the packet based on all signals received by the n antenna branches after the packet detection.
According to the second aspect of the present invention, there is provided a diversity receiver having n (integer not smaller than 2) antenna branches for receiving a packet made up of a preamble field and data field, comprising n first switches for receiving signals respectively received by the n antenna branches and a xe2x80x9cpacket receive pulsexe2x80x9d representing that the packet is being received, and changing an output destination based on the xe2x80x9cpacket receive pulsexe2x80x9d, a second switch for receiving an antenna switching signal and outputs from the n first switches, and when no xe2x80x9cpacket receive pulsexe2x80x9d is output, selecting and outputting the outputs from the n first switches based on the antenna switching signal, a packet detection circuit for receiving an output from the second switch, and when the packet is being received, outputting the xe2x80x9cpacket receive pulsexe2x80x9d, an antenna switching control circuit for outputting said antenna switching signal when the xe2x80x9cpacket receive pulsexe2x80x9d is not input, and a demodulation section for receiving and demodulating all or some of the outputs from the n first switches when the xe2x80x9cpacket receive pulsexe2x80x9d is input.
According to the third aspect of the present invention, there is provided a diversity receiver having n (integer not smaller than 2) antenna branches for receiving a packet made up of a preamble field and data field, comprising (nxe2x88x921) first switches for receiving signals respectively received by (nxe2x88x921) antenna branches and a xe2x80x9cpacket receive pulsexe2x80x9d representing that the packet is being received, and changing an output destination based on the xe2x80x9cpacket receive pulsexe2x80x9d, a second switch for receiving an antenna switching signal and outputs from the (nxe2x88x921) first switches, and when no xe2x80x9cpacket receive pulsexe2x80x9d is output, selecting and outputting the outputs from the (nxe2x88x921) first switches based on the antenna switching signal, one (nxe2x88x92(nxe2x88x921)) antenna branch which is not connected to the first switches, and always directly outputs a reception signal to the second switch regardless of input/non-input of the xe2x80x9cpacket receive pulsexe2x80x9d, one specific demodulation circuit for receiving a signal received by the (nxe2x88x92(nxe2x88x921)) antenna branch via the second switch to always perform demodulation operation, a packet detection circuit for receiving a demodulated signal from the specific demodulation circuit, and when the packet is being received, outputting the xe2x80x9cpacket receive pulsexe2x80x9d, and a demodulation section for receiving and demodulating all or some of the outputs from the (nxe2x88x921) first switches when the xe2x80x9cpacket receive pulsexe2x80x9d is input.
As is apparent from the above aspects, according to the present invention, the n antenna branches are periodically switched to detect arrival of a packet by one demodulation circuit in the demodulation section. When packet arrival is detected, other demodulation circuits are sequentially activated to demodulate reception signals from the n antenna branches. Hence, only one demodulation circuit suffices to operate upon detection of packet arrival, which reduces power consumption. After detection of packet arrival, all the signals received by the n antenna branches can be demodulated and synthesized. Even in communicating a packet having a short preamble field, reception characteristics can be improved by increasing the number of antenna branches.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.